Not only is there no evidence that the Allies tried to 'bribe' Spain to declare war on the Axis, there is strong evidence that the Allies knew any such attempt would be futile for several reasons, not least among which were ideological issues and especially the extreme dislike that the ruling Spanish Falangists had for the Soviet Union.
Further, as Hitler had been unable to bring Spain into the Axis despite their ideological ties, Britain and America realistically had no chance of doing so. Also, a neutral Spain was advantageous to the Allies in that it lessened the likelihood of a German invasion of the Iberian peninsula, which would have posed a severe threat to Gibraltar (see also Why was Operation Felix cancelled?), as well as threatening neutral Portugal.
In 1945, there was an 'offer' (or rather, condition) that countries could not become members of the new United Nations unless they declared war on one or more Axis powers by March 1st, 1945 but, in Spain's case, this offer did not apply. Not until the very end of the war in Europe did Franco consider declaring war on an Axis power (Japan - details below); in the end, though, Spain remained neutral as the Allies made it clear that there was nothing for Spain to gain from declaring war on an Axis power so late in the war.
Prior to the German invasion of the Soviet Union in June 1941, Britain and her empire were strong enough to keep Spain out of the war (by threatening to throttle the Spanish economy and invade the Canary Islands), but not strong enough even to force Franco away from a policy of pro-Axis neutrality (never mind to actually induce her to join Britain). Further, it was essential not to give Germany any compelling reason for invading Spain as
At the end of June 1940 German tanks had reached the border between
France and Spain. Many in the British government thought the Germans
had an open road to Gibraltar....
Source: Jerrold Michael Packard, 'The European neutrals in World War II' (thesis, 1989)
In addition, (thanks here to Pieter Geerkens for his comment) Portugal and Spain were keeping each other neutral through a series of treaties. Thus, even though Portugal was a long-term ally of Britain, the Portuguese remained neutral in WWII as it would not benefit the Allies if this balance on the Iberian peninsula were disturbed to the extent that Germany decided to intervene militarily.
The Allies were also aware that, following the Spanish Civil War, Franco's Spain was economically weak and thus reluctant to involve itself militarily on either side. This weakness also potentially made Spain both an easier target for invasion for Hitler and a weak ally for Britain and America. Thus,
The United States and Britain joined in a continuing effort to keep
Franco’s Spain out of the War by providing essential exports like
gasoline and grain to prop up the Spanish economy, which had been in a
state of collapse since the end of the Spanish Civil War.
Source: Allied Relations and Negotiations With Spain
Following the German invasion of the Soviet Union, the ideological hatred that the Falangists had for the Soviets made it even less likely that Franco's regime would join the Allies. In fact, it led to the formation of the Blue Division, a volunteer force to fight the Soviets:
Recruiting began with a massive demonstration in central Madrid,
during which Serrano declared that “Russia is guilty” of beginning the
Spanish Civil War, murdering José Antonio, and otherwise contributing
to the destruction of Spain’s economy and prospects. Tens of thousands
of Falangists, university students, soldiers, and others wanted to
join the unit...
Source: Wayne H. Bowen, 'Spain during World War II' (2006)
As far as Stalin was concerned, the feeling of hatred was mutual and America's subsequent entry in to the war did nothing to lessen Spanish-Soviet antipathy (although it did eventually help tip Spain towards a less pro-Axis stance. In fact, the possibility of bribing Spain to join the Allies was considered so remote that, at a series of meetings in January 1943,
the allies were prepared, at least on a planning basis, to consider
their own invasion of Spain in order to ensure access to the
Source: Allied Relations and Negotiations With Spain
The closest that Spain came to declaring war on an Axis power came near the end of the war when
...Franco briefly considered
joining the Allies. The precipitating event was an attack on Spaniards
by Japanese troops evacuating Manila in February 1945.... on February 12, 1945,
several hundred Japanese soldiers fired on Madrid’s consulate in
Manila, and also set several buildings on fire, in an attempt to
strike at the more than one hundred people taking refuge there.
As a result,
The Spanish government...immediately withdrew its diplomatic protection over
Japanese assets....Spain also lodged a very stern diplomatic protest
at the Japanese embassy in Madrid, demanding moral and financial
satisfaction for the crimes committed against Spanish citizens and
property....Japan did not have answers for Spain, and so on April 12,
Franco broke diplomatic relations between the two nations...
However, the British and American ambassadors in Madrid made it clear to the Spanish government that a declaration of war would not have any benefits for Spain. Further,
Franco decided against entering the war at
this moment because he did not want to become an ally of the hated
Soviet Union, expected to enter the military conflict against Japan
soon after the German surrender.