I disagree with both my colleagues. Expenses of purchasing investments, including interest on money used to buy investments, is still an eligible deduction. It doesn't matter if the credit card bank calls it a "processing fee", it's still a form of interest. However, the interest deduction is limited to the investment income paid. For example, if you pay 3% interest in 2025, but don't receive the bond interest income until 2026, you must list the interest as an expense on your 2025 return and it will carry forward, to be used as a deduction when you realize the income.
https://turbotax.intuit.com/tax-tips/investments-and-taxes/what-are-deductible-investment-interest-e...
https://www.irs.gov/forms-pubs/about-form-4952
https://www.irs.gov/publications/p550
