As posed, the answer to the question “Did the USSR attempt to broker peace negotiations between Japan and USA?” is no, they did not.
Though the story is a little convoluted and often fraught with presentation of sippets depending on what side of questions surrounding the eventual surrender of the Japanese one takes up.
The Japanese and the Soviet Union signed a five year non-aggression pact on 13 April 1941. On 5 April 1945 the Soviets denounced the pact, thereby invoking the pact’s one year non-intention to renew. See the 13 April 1945 OSS analysis of this action here. Interestingly the report also describes Japanese proposed concessions, similar to, but certainly not as extensive as the Western Allies advanced at the Yalta Conference, to perhaps sweeten a renewal. Some discussion of the concessions also appears in the communications between Japan’s Foreign Minister Togo Shigenori and the Japanese Ambassador to the Soviet Union Sato Naotake. The US was quite aware of the contents of the messaging back and forth between these two as the Japanese diplomatic code had long been broken and the fruit thereof referred to as MAGIC.
In truth, the Soviets had no desire to see the war end without their own chance to gather up the spoils of a Japanese capitulation. There were certain agreements from the Yalta and Potsdam conferences that in return for the Soviet Union entering the war against the Japanese agreed to a Soviet sphere of influence in Manchuria, annexation of the southern half of Sakhalin Island and the Kurile islands, control of Port Arthur, and control over the Manchurian railroad. These agreements essential returned to the Soviet Union all that had been lost by Imperial Russia in the 1905 Russo-Japanese War. See here.
One might note that the Potsdam Declaration was issued by the United States, Great Britain, and China. The Soviets, while certainly present at Potsdam and very interested, were not a party. They were not belligerents in the war with Japan and thus had not a horse in the race. Once they declared war on the Japanese, they simultaneously jumped on the Potsdam bandwagon and supported fully. Knowing that they were planning on entering the war with Japan within 90 days of the German unconditional surrender, it is patently obvious that absent a “we will fully accept unconditional surrender” from the Japanese in their peace feelers, the Soviets had an opportunity to simply lead Japan down a primrose path until they, the Soviets, could enter the war and achieve all that was promised at Yalta, what was being offered by the Japanese in their feeble negotiations, and then some without having to negotiate further any terms with the Japanese.
Some like to point to a MAGIC intercept of 12 July 1945 as instructions from Togo to Sato to firm up in his meetings with the Soviets Japan’s desire to end the war where he says:
“We are now secretly giving consideration to the termination of the
war because of the pressing situation which confronts Japan both at
home and abroad. Therefore, when you have your interview with Molotov
[in accordance with previous instructions] you should not confine
yourself to the objective of a rapprochement between Russia and Japan
but should also sound him out on the extent to which it is possible to
make use of Russia in ending the war.”
That’s all well and good, Togo is instructing Sato to broach the subject of facilitating an end to the war; though, an end to the war, not the unconditional surrender demanded by the Western Allies. The Soviets are keyed on the unconditional surrender part. As long as the Japanese do not publicly announce an unconditional surrender, they, the Soviets, knew they need do nothing but sagely nod their heads and continue moving their armies east and planning the August Storm offensive for early August.
Togo firms up the no discussion of unconditional surrender position in a message to Sato on 13 July 1945 in which he says:
“His Majesty the Emperor, mindful of the fact that the present war
daily brings greater evil and sacrifice upon the peoples of all
belligerent powers, desires from his heart that it may be quickly
terminated. But so long as England and the United States insist on
unconditional surrender the Japanese Empire has no alternative but to
fight on with all its strength for the honor and existence of the
And in a 17 July 1945 message, however, Togo further imparts to Sato:
“The Emperor himself has deigned to express his determination and we
have therefore made this request of the Russians. Please bear
particularly on mind, however, that we are not asking the Russians’
mediation in anything like unconditional surrender.”
And on 21 July 1945 Togo communicates to Sato:
“With regard to unconditional surrender (I have been informed of your
18 July message) [In that message Sato advocated unconditional
surrender provided the Imperial House was preserved] we are unable to
consent to it under any circumstances whatever. Even if the war drags
on and it becomes clear that it will take much more bloodshed, the
whole country as one man will pit itself against the enemy in
accordance with the Imperial Will so long as the enemy demands
unconditional surrender. It is in order to avoid such a state of
affairs that we are seeking a peace which is not so-called
unconditional surrender through the good offices of Russia. It is
necessary that we exert ourselves so that this idea will be finally
driven home to the Americans and the British.”
This would seem to play right into the Soviet’s hands of making no effort to facilitate ending the war. In fact, Stalin, at Potsdam, tells Truman that the Japanese have contacted them but have rejected unconditional surrender. This should leave one to believe that Sato is repeating Togo's “no unconditional surrender” message to the Soviets who, in turn knowing the Western Allies’ firm position, do nothing, lift not a finger, thus ensuring their piece of the pie at the end. The clock keeps ticking.
So, no, with the Japanese stonewalling the issue of unconditional surrender and the Soviets’ own self-interest in the outcome if they enter the war, there was no attempt to broker a peace between the Japanese and the Western Allies.